This invention relates to underwater transducers, and more particularly to improvements in projectors and hydrophones of the flexural type, in which a layer of active transduction material, such as piezoceramic or rare earth magnetostrictive material, is bonded to a passive substrate material, for example brass or steel, which serves as a structural support for the active transduction material.
In a typical flexural underwater transducer, the active layer is coupled to the surrounding water through a sound-conducting material, while the substrate is backed by an enclosed gas, usually air. The active transduction layer is permanently fixed to the substrate by a bonding material such as high strength epoxy. Because the active layer is fixed to the substrate, the bonded pair of layers, when flexed, have a single neutral plane, i.e. an internal plane on one side of which the materials are in compression and on the other side of which the materials are in tension. Since the substrate is ordinarily stiffer than the active layer, the neutral plane is located within the substrate layer.
When the pair of layers is bent, by hydrostatic pressure, in such a way that the face of the active layer which is coupled to the surrounding water is concave, the active layer is in compression, while the substrate is partly in compression and partly in tension.
The active layer is capable of withstanding a limited amount of compressive stress, e.g. up to 30,000 psi in the case of a piezoceramic, but very little tensile stress. In the case of piezoceramic active layer, tensile stresses greater than about 1000 psi are likely to cause damage.
The acoustic performance of the active layer tends to degrade with increasing compressive stress. Therefore, ideally, the compressive stress in the active material should be only enough to compensate for dynamically induced stress. In the case of a projector, the compressive stress should only be enough to prevent the peaks in the sinusoidal dynamic stress, induced in the material by the electrical driving signal, from exceeding the tensile limit. In a hydrophone, the compressive stress in the active layer, caused by static pressure, adversely affects its piezoelectric parameters. In addition, over time, compressive stress causes aging degradation. Thus, the compressive stress in the active layer should be limited to an amount less than that which will cause the active material to degrade in sensitivity. In a conventional flexural transducer in which the transduction material is bonded to the substrate, it has been generally necessary to place the transduction layer under more compressive stress than was desirable, at least in deep water applications.
The design of a typical flexural transducer, therefore, is a compromise between mechanical ruggedness and acoustic performance.
Another problem with typical flexural transducers, and in particular electrically driven projectors, is that, in some cases, their active transduction layers are pre-stressed in compression. The reason for this is to prevent peaks of a large-magnitude electrical driving signal from driving the material into excessive tension, i.e. more than 1000 psi. The built-in compressive stress in the active layer tends to degrade the acoustic properties of the device over time. Therefore, in the case of a device which typically has a long shelf life before usage, degradation of performance can occur even before the device is placed into service.
Another problem in typical flexural transducers is that differential thermal expansion and contraction of the bonded active and passive layers can cause shear stresses in the layers.
Where non-metallic passive layers, formed of materials such as glass fiber or graphite composites, are used, they are typically prestressed in order to insure proper operation when submerged to operating depths. The prestressing of the non-metallic layer can cause material creep, which can have a detrimental effect on the mechanical relationship between the rigidly bonded active and passive layers.